PETROGENESIS OF GABBRO FROM ARCTIC GAKKEL MID-OCEAN RIDGE

Petrologic evolution under mid-ocean ridges (MORs) has long been a major means to study the complex structure of the oceanic crust. Gakkel Ridge, the Arctic mid-ocean ridge, is especially interesting with its unique “ultraslow” spreading rate. Geochemical studies of Gakkel Ridge basalts and peridotites have shown the primitive magma to be created by a low degree of partial melting and a heterogeneous parental mantle (Hellebrand et al., 2002). However, the extremely thin seismic “Layer 3”, correlated generally with gabbro and its properties are not yet known.

We studied the geochemistry of gabbros recovered from the AMORE program (Arctic Mid-Ocean Ridge Expidetion, 2001), using Electron Probe Microanalyzer (EPMA) from Rice University and LA-ICP-MS from University of Houston. The results of the CPX Mg# versus anorthite content (An%) of the studied gabbros show the lowest trend in An% compared to global gabbros. MELTS models of isobaric fractional crystallization of average Gakkel primitive basalt indicates that the solid line of descent (Mg# CPX vs An% Pl) matches with our results quite well. This implies that the primitive melts creating the upper and the lower oceanic crust in the Gakkel Ridge are quite similar. Such models indicate that both are generated with about 7.5% of partial melting, assuming average DMM composition (Workman & Hart 2005). Solid line of descent models derived from gabbros can thus be used to predict (or infer) the thickness of the oceanic crust. The crustal thickness calculated by the Gakkel gabbros is about 2 – 4 km in accordance with predictions made with the basalt and the seismic data.

Reference:

Hellebrand, E., Snow, J. E. & Mühe, R. (2002). Mantle melting beneath the Gakkel Ridge (Arctic Ocean): abyssal peridotite spinel compositions. Chemical Geology 182, 227-235.

Workman, R. K. & Hart, S. R. (2005). Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters, 231, 53-72.